This invention relates to a particular class of radar systems, more specifically radars for detecting high velocity airborne targets by "Doppler tolerant" types of radar pulse modulation. More particularly, the invention relates to radar pulse compression of the target returns of such radars by digital finite impulse digital filters whose coefficients are the conjugates of the corresponding of the "Doppler tolerant" modulation algorithm of the radar pulse.
Early radar systems employed narrow transmitter pulses to provide fine range resolution of target returns. Such primitive systems required extremely high peak power to achieve long range detection. In addition, narrow transmitter pulses perform very poorly with regard to surface and weather clutter.
To alleviate these problems, pulse compression techniques have been incorporated in radar systems. In general, such techniques involve the use of radar pulses of long duration but limited peak power. Prior to transmission, the long pulse is modulated by a predetermined amplitude, phase or frequency varying function, and the energy of the resulting return pulse is compressed into a narrow pulse of fine range resolution by a corresponding correlation function. Thus, long range detection is facilitated by the long pulse duration,
Since the introduction of pulse compression techniques to radar systems, improvements in radar capabilities have largely focused on the development of sophisticated pulse modulation functions. One particular problem which has been addressed by such functions is the detection of high speed aircraft which have increased in number both in military and civilian applications. Studies undertaken in the mid-1970's have indicated that broad Doppler spectrum tolerance--i.e., the ability to compress and detect returns from multi-Mach airborne targets--is exhibited by long pulses of linearly varying frequency. More recently, promising results have been obtained using another family of modulating functions which produce pulses of hyperbolically varying frequency.
It is especially desirable, in detecting and tracking high speed aircraft, to process the reflected radar pulses immediately. The use of long, Doppler tolerant pulses, however, exacerbates the problem of real time pulse compression, requiring a substantial amount of hardware to realize the necessary processing throughput which may, in some instances, require billions of arithmetic operations per second. A review of present available techniques for Doppler tolerant pulse-compression indicates that each exhibits one or more important drawbacks.
With regard to wholly analog techniques, dispersive delay lines for processing of long pulses--up to one 20 millisecond in duration--are massive and therefore impractical for most applications. Saw devices, representing another analog approach, are limited to pulse widths of less than 50 or 60 microseconds.
Compression of linear or hyperbolic frequency modulated (LFM, HFM) radar return pulses has heretofore centered on the use of discrete analog devices commonly known as charge coupled devices (CCD's) or charge transfer devices (CTD's). Radar return samples are sequentially stored in analog form in each of a plurality of capacitive elements making up the CCD. To achieve the same result in a completely digital manner, with similar return sample amplitude resolution each CCD register would have to be replaced by several digital shift registers, one for each required bit of sample amplitude resolution. CCD registers accommodating as many as 512 data points have been implemented for compressing linear frequency modulated (LFM) pulses with bandwidths in excess of 1.25 megahertz.
While present CCD filters have provided a workable solution to the LFM pulse compression problem, several problems remain. In particular, CCD's are bandwidth limited and exhibit poor dynamic range (55 dB), making full realization of the theoretical benefit of LFM radar difficult in practice.
Improvements in digital hardware have stimulated interest in wholly digital radar pulse compression techniques. Some of the earlier developments of such techniques relied on specially encoded pulses designed for efficient recognition. These techniques have proven unsatisfactory with respect to Doppler tolerance due to the discrete phase and frequency jumps inherent in the digitally encoded pulses.
U.S. Pat. No. 4,006,351, issued to Constant suggests that frequency domain signal analysis techniques may be applied to digital radar processing. As indicated, Discrete Fourier Transform (DFT) and Fast Fourier Transform (FFT) filters may be employed as clutter filters in radar systems, but the implementation of a frequency domain filter for long LFM and HFM pulses would require a substantial number of currently available digital hardware elements.
Another general type of digital filter operates in the time domain, thus avoiding the necessity of Fourier and inverse Fourier analysis. Such filters have found application in audio and data transmission systems, as exemplified by U.S. Pat. No. 3,912,917, issued to Nussbaumer, and for radar clutter cancellation, as disclosed in an article by S. Haykin and C. Hawkes entitled "Adaptive Digital Filtering for Coherent MTI Radar," Information Sciences, Vol. 11, No. 4, 1976, pages 335 to 359. Again, however, because of the high frequencies and corresponding high sample rates associated with radar signals, digital time domain filters have not been suggested for the more computationally intensive task of real time compression of linear or hyperbolic FM radar pulses.